The start some time ago of digital satellite broadcasting has been followed by widespread acceptance of digital satellite broadcast receivers by the general public. A large number of broadcast stations are offering digital satellite broadcast services. The digital satellite broadcast receiver is typically designed to let any one of multiple channels be selected from among the numerous offerings.
FIG. 5 is a block diagram outlining an internal structure of a conventional digital satellite broadcast receiver.
As illustrated, the digital satellite broadcast receiver includes a front end section 51, a demultiplexer 52, a decoder 53, and a system controller 50 that controls these components.
The front end section 51 acts on a setting signal supplied from the system controller 50, the signal designating transmission specifications and other technical details. In accordance with the supplied setting signal, the front end section 51 receives through a parabola antenna (not shown) a carrier (reception frequency) designated by the signal. From the carrier, the front end section 51 acquires a transport stream (TS) through a Viterbi decoding process and an error correction process.
As is well known, the transport stream as per the digital satellite broadcast criteria is constituted by signal data in compressed form, multiplexed with diverse kinds of additional information. The compressed data are prepared by compressing video and audio signals of a plurality of broadcast programs in accordance with MPEG-2 (Moving Picture Experts Group Layer 2). The additional information, to be inserted by broadcast stations, includes PSI (program specific information) accommodating tables such as PAT (program association table) and PMT (program map table), and SI (service information, serving as program schedule information).
The front end section 51 obtains PSI (program specific information) packets from the transport stream (TS) in order to update channel select information. From the transport stream, the front end section 51 also acquires a component PID (program ID) on each of the available channels and forwards the acquired component PID's illustratively to the system controller 50. The system controller 50 utilizes the received PID's in processing the received signals.
In keeping with a filtering condition established by the system controller 50, the demultiplexer 52 filters out necessary TS packets from the transport stream supplied by the front end section 51, in a manner to be described later. Illustratively, the demultiplexer 52 obtains regarding a target broadcast program two kinds of TS packets: TS packets of video data compressed in the MPEG-2 format, and TS packets of audio data compressed also in the MPEG-2 format. The compressed video and audio data thus acquired are output to the decoder 53.
The decoder 53 is made up of a video decoder and an audio decoder. The video decoder decodes compressed video data in accordance with the MPEG-2 format for output. The audio decoder decodes compressed audio data in keeping with the MPEG-2 format and outputs the decoded audio data in synchronism with the video data output. The compressed video and audio data that have been input are thus decoded by the video decoder and audio decoder respectively.
The video and audio data that have been decoded and synchronized as outlined above are output for reproduction purposes by the decoder 53.
In operation, the conventional digital satellite broadcast receiver of the above-described structure may receive a channel select request from a user manipulating the receiver. In such a case, the steps outlined below are typically carried out.
FIG. 6 is a flowchart of steps in which the conventional digital satellite broadcast receiver selects a channel.
In step S1001, the system controller 50 of the receiver checks continuously whether a channel select request is made. If the channel select request is found to be made, the system controller 50 goes to step S1002 and recognizes the channel number of the requested channel.
In step S1003, NIT (network information table) is received on the basis of PAT (program association table). As is well known, NIT includes channel-related physical characteristic information (e.g., satellite orbit data, polarized waves, and frequency of each of the transponders associated with digital satellite broadcasts). The received NIT is used in step S1003 as a basis for recognizing the transponder frequency covering the requested channel.
In step S1003, the frequency thus recognized is set anew for the front end section 51. This causes the front end section 51 to receive the transponder frequency covering the requested channel.
In step S1004, PAT is acquired from the transponder frequency received anew by the front end section 51. In step S1005, PMT of the requested channel is received in accordance with information in the newly-received PAT.
In step S1006, a filtering condition is set for the demultiplexer 52 based on PMT. The condition allows the demultiplexer 52 to filter out only the packets of the requested channel from the transport stream of the transponder, whereby stream data are obtained.
In step S1007, SH (sequence header) is detected from the stream data acquired by the demultiplexer 52 in the manner described above. From the detected SH, the demultiplexer 52 recognizes a video format (e.g., standard definition (SD) or high definition (HD)) of the stream data on the requested channel.
In step S1008, SH detected in step S1007 is used as a basis for establishing various settings needed by the decoder 53 to operate in keeping with the relevant format.
In step S1009, the decoder 53 is prompted to execute a decoding process based on the settings. The decoder 53 thereby establishes synchronization between video data and audio data.
In step S1010, the decoder 53 is prompted to output the video and audio data thus prepared. This is how the audio and video data from the channel requested in step S1001 are output.
As described, the conventional digital satellite broadcast receiver performs two stages of processing, i.e., demultiplexing (steps S1002 through S1006 in FIG. 6) and decoding (steps S1007 through S1009), from the time a channel select request is made until an image of the requested channel is displayed.
As can be seen from the foregoing description, the demultiplexing process can take long, as much as hundreds of milliseconds. That is because the process involves carrying out a fairly large number of steps in sending and receiving various signals. Likewise, the decoding process can take hundreds of milliseconds because it is relatively complicated in execution.
As a result, the conventional digital satellite broadcast receiver can take up to one second between the channel select request and the output of video display. This amounts to a substantially long delay in channel selection compared with the typical analog TV set (television receiver) receiving analog broadcasts.
With such behavior, many conventional digital satellite broadcast receivers have often aroused in their users a feeling of awkwardness due to the prolonged channel select time that is considerably longer than that of analog TV sets.